topic Re: Constraining splines in sketch mode in Fusion Design, Validate & Document Forum

Constraining splines in sketch mode

andrewR62MC — Sun, 04 Nov 2018 19:45:45 GMT

Hi, I've started using splines, but don't know how to fully constrain them. Anybody have any thoughts?

Re: Constraining splines in sketch mode

lichtzeichenanlage — Sun, 04 Nov 2018 22:31:20 GMT

The quick way is to right click -> Fix / UnFix it. But this is not parametric and might come with some drawbacks. The "proper" way is to constrain the spline points and handles with dimension and / or constrains.

Re: Constraining splines in sketch mode

Urbanthor — Wed, 09 Sep 2020 11:36:02 GMT

when moving a sketch with splines, I have to constrain both the points and the handles. so one spline needs to be constrained 6 times. ><

Isn't there A better way?

I just spent an ungodly amount of time constraining the splines just so I could coincident the sketch ...

Re: Constraining splines in sketch mode

g-andresen — Wed, 09 Sep 2020 12:34:11 GMT

Hi,

@Urbanthor wrote:
Isn't there A better way?

1. What is the reason that forces you to move a sketch? I do almost all position changes with components and joints.
2. Control point splines need only 2 dimensions per point.

günther

Re: Constraining splines in sketch mode

TrippyLighting — Wed, 09 Sep 2020 19:16:15 GMT

So here's a question that might border on heresy for some folks:

Why would you need to fully constrain and dimension a spline?

Re: Constraining splines in sketch mode

MichaelT_123 — Wed, 09 Sep 2020 23:30:17 GMT

Hi Mr. Guenther Andresen,

generally ... only 3 dimensions per point ... 😉

Regards

MichaelT

Re: Constraining splines in sketch mode

therealsamchaney — Wed, 05 Jan 2022 18:28:19 GMT

Each spline point has a handle (a line connected to the point that controls the curvature at that point) so for every one of these points you'll have to fully constrain this handle line.

To do this you will need to make an X and a Y dimension to constrain the point, then a width and an angle dimension to constrain the handle line.

So, that's 4 dimensions for every point in the spline. If you have some of the points constrained in some other way like with coincident or tangent, you won't need all of these dimensions for those points.

You can also choose to only constrain the X and Y of the points but leave the handles unconstrained so you can adjust the curvature still.

Re: Constraining splines in sketch mode

patmat2350 — Fri, 22 Jul 2022 17:46:20 GMT

"You can also choose to only constrain the X and Y of the points but leave the handles unconstrained so you can adjust the curvature still."

This has caused be problems! I now understand that the handles require constraints, but in the past, simply constraining X-Y coords for points would turn the spline black, fooling me into thinking the spline was constrained. Silly me for thinking that!
So, add a line segment tangent to the end of the spline (or parallel to the end handle, same thing I think?), and adjust the distal line end point, and Shazzam! The spline goes out of shape. @$%$#!
I do see that if at least one handle has been adjusted, the spline turns blue again.
Suggestion to the UI team: ANY unconstrained spline should be blue!

Adding to the confusion: Sketches containing these pseudo-constrained splines get the red padlock ( https://forums.autodesk.com/t5/fusion-360-design-validate/what-does-a-red-quot-lock-quot-icon-mean-on-a-sketch-icon/td-p/9134607 ).

When working on boat hulls, I may have a score of splines having a score of points each. That would be a lot of constraints to add.

Re: Constraining splines in sketch mode

TrippyLighting — Fri, 22 Jul 2022 18:14:12 GMT

@patmat2350 wrote:

When working on boat hulls, I may have a score of splines having a score of points each. That would be a lot of constraints to add.

It would indeed be, but that many fit points for a spline is also a surefire way to create splines with horrible curvature. Having as many splines as you have in the image you posted is equally a surefire way to create surfaces with bad curvature.

Re: Constraining splines in sketch mode

patmat2350 — Fri, 22 Jul 2022 18:23:11 GMT

I agree it's an issue. But I'm recreating historic vessels here, and my own simplified splines will rarely follow the original lines well enough to satisfy me.

Side note: The data tables ("Table of Offsets") defining these shapes, at least in the UK & USA, were defined in feet/inches/eighths of inches. The old draftsmen did good work, but I often find that some points need tweaking within the rounded eighths to achieve decent fairness.

I'm afraid I don't know much about control point splines... I wonder if they would do a better job in fairing the data?

Re: Constraining splines in sketch mode

TrippyLighting — Fri, 22 Jul 2022 18:44:35 GMT

@patmat2350 wrote:

I agree it's an issue. But I'm recreating historic vessels here,

It is a mathematical issue. The NURBS math doesn't care how old the vessels are 😉

@patmat2350 wrote:

... but I often find that some points need tweaking within the rounded eighths to achieve decent fairness...

How do you judge fairness of a spline, or surface ? Are you using the tools in the inspect menu?

For example the curvature comb on splines of edges, or the Zebra stripes or IsoCurve analysis on surfaces?

@patmat2350 wrote:

...I'm afraid I don't know much about control point splines...

The old draftsmen and boat builders bent wooden sticks around pins to create smooth curves> These "wooden sticks" were the first splines. Then much later mathematical formulae were developed that resembled the behavior of such sticks. Those Bezier curves also were called Bezier splines. Fitpoint splines in Fusion 360 are 5-degree multi-span B-Splines. The more fitpoints you add to such a spline, the more complex that mathematical monster becomes.

While ultimate curvature quality is unlikely needed for projects such as these, I would really limit the number of fit points and number of splines and try to get the overall shape of the boat correct.

@patmat2350 wrote:

I wonder if they would do a better job in fairing the data?

No, because you can only manually control/move the CVs (control vertices.) They are simply easier to handle when creating gentle curves.

Other CAD software sometimes includes algorithms to fair splines for a best fit through data points. This does not exist in Fusion 360. If you want to go down that rabbit hole I would look at software specialized for boat builders. But I doubt that's really what is needed.

Re: Constraining splines in sketch mode

patmat2350 — Fri, 22 Jul 2022 20:06:55 GMT

Math don't care... but I do!

And it's combs on splines. I confess, it's hard to appreciate the meaning of the comb magnitude... or to get the comb as smooth looking as I would like... but on, for example, this convex curve, I'm happy to get all the comb on one side with no curvature reversals.

Re: Constraining splines in sketch mode

TrippyLighting — Fri, 22 Jul 2022 22:55:39 GMT

@patmat2350 wrote:

Math don't care... but I do!

When you're using computer software math and algorithms have the last word, no matter how much you care.

Suit yourself.

Re: Constraining splines in sketch mode

patmat2350 — Tue, 09 Aug 2022 04:27:40 GMT

"...but that many fit points for a spline is also a surefire way to create splines with horrible curvature. Having as many splines as you have in the image you posted is equally a surefire way to create surfaces with bad curvature."

Oh, I'm happy with my abundance of spline points...

Re: Constraining splines in sketch mode

lichtzeichenanlage — Tue, 09 Aug 2022 07:54:59 GMT

That's a nice model. But unfortunately a rendering doesn't gives away to much information. Can you show an e.g. zebra analysis of the hull? As you can see, both bodies do look fine in the design view but not in the zebra analysis. And I only added 3 additional rail profiles.

And the bodies are nearly identical

Re: Constraining splines in sketch mode

wersy — Tue, 09 Aug 2022 08:15:06 GMT

@patmat2350 schrieb:
Oh, I'm happy with my abundance of spline points...

Are all combs really that smooth?
Very rarely do you get such exact profiles for a boat.
You can't tell from the picture how perfect the hull is. Anyway, it looks very good, a nice job.
What kind of boat is this? It looks like some kind of lifeboat.
Can you post a link to it?

Re: Constraining splines in sketch mode

lichtzeichenanlage — Tue, 09 Aug 2022 12:12:56 GMT

CurvatureMap is great to show surface problems too:

Re: Constraining splines in sketch mode

patmat2350 — Tue, 09 Aug 2022 20:40:53 GMT

"Are all combs really that smooth?
Very rarely do you get such exact profiles for a boat."

First, I really only need to make the render look presentable, I won't be machining a perfect shape from billet or any such.
If a physical model is made, it will be "old school"- cutting frames from wood on a bandsaw, sanding fair (enough), planking, and sanding again.

And no the original data wasn't perfect. The occasional mistake in tabulated data from a 1909 draftsman, and everything rounded to 1/8 inches. So I used curvature combs to refair the individual frames a bit, and added as many rails along the waterlines as possible.

And yes it's a lifeboat for the US Life Saving Service.
https://www.rcgroups.com/forums/showthread.php?4169297-US-Life-Saving-Service-36-Elco-boat

Re: Constraining splines in sketch mode

lichtzeichenanlage — Tue, 09 Aug 2022 22:08:28 GMT

You just want a rough model? Fine. But...

Why run through all the effort and creating so many sketches the goal is a rough model.
Don't say that many curves are a good solution without saying that you don't care about the surface quality. Because this way your post doesn't help other users
I'm not sure if the back of your hull is even convincing in the rendering. But perhaps this is just a strange reflection.

Any chance that you can show us a zebra analyzes or curvature map? Doesn't take much time to do this.

@patmat2350 wrote:
"Are all combs really that smooth?
Very rarely do you get such exact profiles for a boat."

First, I really only need to make the render look presentable, I won't be machining a perfect shape from billet or any such.
If a physical model is made, it will be "old school"- cutting frames from wood on a bandsaw, sanding fair (enough), planking, and sanding again.
And no the original data wasn't perfect. The occasional mistake in tabulated data from a 1909 draftsman, and everything rounded to 1/8 inches. So I used curvature combs to refair the individual frames a bit, and added as many rails along the waterlines as possible.
And yes it's a lifeboat for the US Life Saving Service.
https://www.rcgroups.com/forums/showthread.php?4169297-US-Life-Saving-Service-36-Elco-boat

Re: Constraining splines in sketch mode

patmat2350 — Wed, 10 Aug 2022 04:39:39 GMT

It's not a rough model- it's very true to the original.
I don't give a hoot about the analytical capabilities of some striped horse- the shape is correct, and smooth enough to appear fair in images. Maybe it could be smoother, who cares? If it's smooth but wrong because it has only three sections and two rails, it's of no use to me.
And I assure you, my boat's butt is near perfect- you're likely confused by the turn of the bilge versus the deadwood area.